Motors that are powered by propulsive fluids are very common and useful. One use for such motors is for powering a pump. Propulsive fluids include air, hydraulic oil, and water and are typically pressurized by a pump.
There are at least two general types of fluid driven motors. "Continuous" motors, e.g. rotary vane motors, include sealing members that move in a continuous fashion. "Reciprocating" motors, on the other hand, include sealing members that reciprocate or oscillate between a first position and a second position. A very common type of reciprocating motor has a single or double sided poston.
A reciprocating motor typically includes a sealing member in a chamber that divides the chamber into two subchambers. In the single-sided piston embodiment, a pressurized fluid in the first subchamber causes the piston to move in a first direction and a spring returns the piston in a second direction. In the double-sided embodiment, the propulsive fluid is on both sides of the piston and the piston moves in response to a differential pressure across it. Reciprocation of the double-sided reciprocating motor is caused by pressurizing the first subchamber while venting the second subchamber, then venting the first while pressurizing the second. The pressurizing and venting cycle is repeated to cause the piston to reciprocate. The frequency at which the motor reciprocates typically depends on the flow rate of the propulsive fluid, among other things. This frequency can be termed the "motor frequency".
In order to reciprocate the sealing member, a valve system is normally occupied to the motor. The valve system places the first subchamber in fluid communication with a higher pressure than the second subchamber, causing the sealing member to move, including simply deforming as in the case of a diaphragm. The valve system switches when the sealing member reaches the end of its "stroke" so that the first subchamber is vented and the second subchamber is pressurized. The valve system must therefore reciprocate between a first position and a second position at the motor frequency.
Additionally, the valve system should preferably have the characteristic of "over-center" or "snap" action. That is, the valve system should remain in a first position until the sealing member reaches the end of its stroke and then snap to a second position. Otherwise, if it does not snap between its first and second positions, the valve system can reach an equilibrium condition in which the sealing member is in an intermediate static state and the valve system is likewise in an equilibrium state. In such a set of conditions, the sealing member ceases to move and the motor "stalls."
One method to accomplish snap action involves the use of a linkage that includes springs. Such systems typically involve several moving parts and reliable and repeatable performance depends on the springs which can fail or lose calibration.
Magnetic fields have also been used to achieve a snap acting valve system. Magnetic forces drop off rapidly with increasing distance from the source of the field. This property makes magnets useful for snap action mechanisms as discussed below. Prior art designs for reversing mechanisms that incorporate magnets suffer from various disadvantages, however, as will also be discussed below.
One class of devices, represented by U.S. Pat. No. 3,299,826, couples a member that moves in synchrony with the reciprocating sealing member to a magnet with a spring. That is, the member is acted on by a magnetic force and by a spring, the forces being in opposition. As the sealing member moves the spring force increases until it is sufficiently large to overcome the magnetic force, and the spring causes the member to snap to a second position, thus causing a valve system to switch to a second state to force the sealing member in the opposite direction. One problem with this type of system is that if the magnet dissipates in strength or decay the sealing member will tend to have a shorter stroke, since the spring force will overcome the magnetic force at an earlier point in the sealing member's stroke. Another problem with this type of system involves the springs themselves. If the spring constant changes as the spring ages, the valve system will again have a different switching point with respect to the stroke of the sealing member. Generally, as the spring changes the snap action will have a different characteristic. In fact, the spring might fail altogether due to repeated cycling.
Another class of magnetic reversing mechanisms is represented by U.S. Pat. No. 3,304,126. In this type of mechanism springs are not used. A core of a directional control valve is acted on by magnetic forces and fluid pressure forces. The core is in fluid communication with the subchambers and when the differential pressure across the core is larger than the magnetic forces, the core will move from a first to a second state. One problem with this type of mechanism is that it is relatively complex. Additional porting is needed to communicate the pressures in the subchambers to opposing surfaces on the valve core. Also, as in the case of the magnet/spring mechanisms discussed above, the switch point of the valve core is dependent on the strength of the magnets. If the magnets decay, the valve core will switch at an earlier point thus decreasing the stroke of the sealing member.
These devices have not been found to be entirely suitable. The present invention addresses many of the problems discussed above. Specifically, the present invention provides a directional control valve that includes a snap acting magnetic reversing mechanism. The valve may be used with a reciprocating fluid-driven motor which includes a member that is driven by the motor's sealing member. The driven motor includes a magnetic area, and a valve core also includes a magnetic area. Reciprocative movement of the sealing member at a motor frequency causes the driven member to also reciprocate at the motor frequency which further causes, through magnetic coupling, reciprocative movement of the valve core. The valve core snaps from a first position to a second position and back again due to the characteristics of the magnetic interaction between the aforesaid magnetic areas.
The reversing mechanism that is the subject of this application does not include springs to effect the snap action; therefore, the problems associated with the springs, as discussed above, are substantially eliminated. Further, the point at which the valve core switches, with respect to the stroke of the sealing member, is not heavily dependent on the strengths of the magnets in the present invention. Thus, the stroke of the motor should remain constant throughout the life of the motor.